In document processing devices, accurate and reliable registration of the substrate media as it is transferred in a process direction is desirable. Even a slight skew or misalignment of the substrate media through an image transfer zone can lead to image and/or color registration errors. Such registration errors can occur as the substrate media passes through the nips.
Document processing devices typically include one or more sets of nip assemblies used to transport substrate media, such as sheets of paper, through the device. A nip assembly provides a force to the sheet as it passes through a nip to propel it through the document processing device. A nip assembly typically includes a drive wheel and an idler wheel in rolling contact with the drive wheel. One or more sets of drive wheels and idler wheels may be longitudinally aligned in order to form the nip therebetween. The driving wheel and the idler wheel may be urged together by a biasing device which in turn creates the nip force. The nip force is required such that the wheels properly engage the sheet as it passes through the nip. This nip force must be significant enough in order to eliminate slipping between the drive wheel and the sheet.
When a sheet being transported through the document processing device first engages the nip, the drive wheel and idler wheel are in rolling engagement with each other. As the sheet engages the wheels, at least one of the idler and drive wheels typically moves against the nip force in order to permit the sheet to enter the nip. The entering of the sheet, especially thick sheets, into the nip results in nip disturbances which negatively affect sheet registration. When a sheet enters a nip, the sheet must perform work in displacing the wheel of an amount equal to its thickness multiplied against the nip force. This work needs to be performed in the time it takes the sheet to fully enter the nip. The work required to move the wheel originates from a decrease in kinetic energy, i.e., speed, of the rotating nip components. The controls used to regulate the nip velocity typically cannot effectively mitigate the nip disturbances. Registration of the sheets, therefore, is compromised.
Accordingly it would be desirable to provide a substrate media transport system having nip assemblies that reduce the disturbance caused by substrate media entering the nips.